The invention concerns a procedure for creating an inert gas atmosphere with constant composition of nitrogen and carbon dioxide in a refrigerated container which is equipped with a diesel engine and a refrigerator and in which a low oxygen content is maintained by means of a limit value control for oxygen.
It is known from several scientific investigations that agricultural produce and animal products, as well as flowers and plants can be stored much longer in a fresh condition in cooled rooms, if the natural atmosphere is replaced with an inert gas atmosphere of nitrogen and carbon dioxide. In most cases a low oxygen content is also advantageous. The optimum gas mixture compositions for the individual products vary, but in general, it can be stated that the contents of carbon dioxide and oxygen are low, amounting to a maximum of ten percent by volume. It is only in the case of transport of fresh meat that the carbon dioxide content may advantageously be increased to fifteen percent by volume.
In large stationary storage rooms, such inert gas atmospheres can be created and maintained with relative ease, since trained personnel is available in these places. However, it is also desirable that similar inert gas atmospheres could be maintained in refrigeration containers for the transport of such agricultural products as well. There has been no lack of attempts in this direction. So, for instance, U.S. Pat. No. 3,269,133 shows a refrigerated container to which a storage container for liquid nitrogen and a gas bottle for carbon dioxide are attached on the outside. Lines lead from these storage containers into the interior of the container. The oxygen contents in the atmosphere are analyzed by means of an oxygen measurement device. When the oxygen content is too high due to air penetrating through leakages in the walls, a control impulse is generated for feed of gaseous nitrogen from the liquid nitrogen container. The nitrogen feed is terminated as soon as the preset oxygen range has been reached again. Independently thereof, the carbon dioxide content is monitored by means of a measurement device for carbon dioxide. If the carbon dioxide content sinks below the predetermined value, the carbon dioxide feed from the gas bottle is released by means of control impulses until the predetermined carbon dioxide content has again been reached. Refrigerated containers have also been used, where the containers for nitrogen and carbon dioxide are located in separate compartments in the interior of the container, as are the refrigerator, the diesel engine and the fuel tank. The standard outside dimensions of the container may be maintained for such arrangements which is a great advantage particularly for the transport. However, measurement and control of the carbon dioxide content have been found unsatisfactory. While there exists a whole series of proven and robust devices for measurement and control of the oxygen content, the corresponding devices for carbon dioxide are complicated and susceptible to malfunction. While this may be acceptable in stationary installations, since trained personnel can be available, this is not the csse with respect to refrigerated containers. In traffic, these are subjected to operational hardships and are serviced by a variety of people with the most varying qualifications. Thus, under practical conditions, refrigerated containers with controlled inert gas atmosphere have not been successfully used.